Remotely extinguishing flames through transient acoustic streaming using time reversal focusing of sound

Acoustic waves are a possible reusable method to extinguish flames. Previous studies have placed the sound source near the flame or have used standing waves to reach large enough acoustic amplitudes to extinguish it. In this study, a new method is explored: using time reversal in a room to focus transient acoustic waves to the flame to extinguish it. The peak acoustic overpressure level needed to extinguish a candle flame in the middle of the room is 191 dB <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mrow></mml:mrow> <mml:mtext>peak</mml:mtext> <mml:mrow></mml:mrow></mml:mmultiscripts> </mml:math> re 20 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>μ</mml:mi></mml:math> Pa when using a frequency range of 300 to 15000 Hz. The sound level at other locations of the room during the focusing was 130 dB. The required peak level is lower when using a less stable flame, or when the flame is near a room boundary. The momentary focus of high-amplitude sound waves subsequently causes acoustic streaming or a flow of air at the flame location that extinguishes the flame. By analyzing high-speed video, it is shown that acoustic streaming extinguishes the flame when using this method, not the acoustic particle displacement. It is also shown that the streaming does not occur when the flame is not present.